LTR-like sequences of human endogenous retrovirus (HERV) and other viruses make up as much as 8% of genome DNA. Of 20 or more kinds of HERVs identified in the human genome, most have mutations in gag, pol and env genes. Some of the genes were detected as being expressed in placenta, teratocarcinoma cell lines, tumors derived from germ cells, breast cancer cell lines, etc.
In particular, the HERV-H gene family is the largest group, including about 100 copies of full-length sequences, 800 to 900 copies of sequences with deletion mutations and about 1000 copies of LTR sequences.
In the HERV-H gene family, it has been reported that HERV-H env gene shows high expression in placenta, skeletal muscle, spleen and thymus, whereas no expression was detected in other normal tissues. Its expression is also detected in tumor cell lines derived from wide variety of tissues (Yi, J-M, Kim, H-M, and Kim, H-S (2006) Tumor Letters vol. 231, pp. 228-239).